A high pressure flow (HPF) subsystem or chamber developed for use with the ENDURA.RTM. PVD system, available from Applied Materials, Inc., is based on a materials processing technology known as "Hot Isostatic Pressing" (HIP). HIP is commonly used in applications such as powder metallurgy and diffusion bonding. During a HIP process, a material deposited on a substrate is subjected to simultaneous application of high isostatic pressure and elevated temperature. Isostatic pressure means equal pressure is applied in all directions by a pressurized processing gas. Argon gas is the most widely used processing gas. Elimination of internal defects such as voids and microporosity is achieved through plastic deformation, creep, and diffusion of the material during the HIP process. The pressure and temperature applied to a material deposited on a substrate depend on the material properties and applications. An HPF process may be applied to fill vias, contacts, and trenches with aluminum at wafer temperatures above about 300.degree. C. and pressures between about 1,000 psi (5.2.div.10.sup.4 Torr) and about 12,000 psi (6.2.times.10.sup.5 Torr).
Via and contact fill capability and low contact resistance are essential for ultra large scale integration (ULSI) interconnect technology. As the device geometries of semiconductors continue to shrink towards the sub-quarter micron regime, it is becoming increasingly challenging to completely fill vias, contacts, and trenches of small dimensions and high aspect ratios. An HPF chamber can achieve complete fill of the vias, contacts, and trenches through application of HIP.
Prior to transferring the wafer to an HPF chamber, the wafer receives a layer of aluminum sputtered thereon at a temperature above 300.degree. C. to achieve large grain size (.about.1 micron). Typical sputtering processes will form an aluminum film which bridges the via, contact, or trench, thereby creating a void within the feature. The wafer is then transferred into an HPF chamber, where it is heated above 300.degree. C. to promote plastic deformation and exposed to high pressure Argon gas at several thousand pounds per square inch (psi). Due to the pressure difference between the inside and the outside of the voids, and the elevated temperature enhancing the flow of the material, the aluminum layer is extruded into the voids. Therefore, the via, contact, or trench holes are completely filed and a void-free metal interconnect is formed.
The design and operation of an HPF chamber is complicated by the fact that it must be compatible with both the high pressure conditions of hot isostatic pressing and the high vacuum conditions associated with integration on a cluster tool. Cluster tools are beneficial for limiting exposure of the wafer and devices being formed thereon to particulate contaminants that may be present in the clean room air. Cluster tools also provide a controlled environment in which a wafer or plurality of wafers may be processed through a number of processes and steps without exposure to the atmosphere. The cluster tool will typically have a transfer chamber through which wafers are transported from a loadlock chamber to one or more processing chambers or other cluster tools. The transfer chamber is a low particle environment maintained at a pressure of about 10.sup.-8 Torr.
An examplary process using the HPF chamber includes the steps of depositing a PVD TiN liner over a substrate having a high aspect ratio via formed therein, depositing a PVD Al layer over the TiN liner, then transferring the substrate into an HPF chamber for exposure to heat and an argon gas source at pressures between about 1,000 and about 12,000 psi.
Elemental aluminum (Al) and its alloys have been the traditional metals used to form lines and plugs in semiconductor processing because of aluminum's low resistivity, superior adhesion to silicon dioxide (SiO.sub.2), ease of patterning, and high purity. However, as line widths decrease and current density increases, materials with better resistivities and better electromigration are being sought. Electromigration is a phenomenon that occurs in a metal circuit while the circuit is in operation, as opposed to a failure occurring during fabrication. Electromigration is caused by the diffusion of the metal due to the electric field set up in the circuit. The metal gets transported from one end to the other after hours of operation and eventually separates completely, causing an opening in the circuit. This problem is sometimes overcome by Cu doping and texture improvement. Electromigration is a problem that gets worse as the current density increases.
Copper and its alloys, on the other hand, have lower resistivities than aluminum and significantly higher electromigration resistance. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increased device speed. However, the primary problems with integrating copper metal into multilevel metallization systems are (1) the difficulty of patterning the metal using etching techniques, (2) the inherent difficulty in filling small vias using conventional PVD processes, and (3) the difficulty in flowing copper by purely thermal processes.
For devices of submicron minimum feature size, wet etch techniques for copper patterning have not been acceptable due to liquid surface tension, isotropic etch profile, and difficulty in over-etch control. A reliable dry etch process is not well developed.
As feature sizes decrease and aspect ratios of features increase, PVD processes have increasing difficulty in achieving good aperture filling or step coverage. Physical sputtering of target material results in target atoms traveling at acute angles relative to the substrate surface. As a result, where high aspect ratio apertures are being filled, sputtered atoms tend to deposit on the upper wall surfaces and bridge the opening thereof before the aperture is completely filled with deposition material. The resulting structure typically includes voids therein which compromise the integrity of the devices formed on the substrate.
Filling of vias, holes, and trenches with copper by HIP is prohibited by the high melting point of copper, which is 1085.degree. C. (1358.degree. K.), and by the low bulk diffusivity of copper. The surface diffusivity of copper is substantially greater than the bulk diffusivity, however bridging across the feature occurs very easily at the high temperatures. HIP processes rely on thermally activated "creep" type plastic deformation of copper which requires an excessively high temperature that would damage underlying layers on the substrate. Without high temperature plastic deformation at a temperature substantially greater than one-half of the melting temperature (.degree.K.), it is expected that a deposited metal layer would not completely fill the submicron features.